It is an object of the present invention to provide an ultrasonic transducer, which is so configured as to reduce the variations in characteristics, thereby to enable the stabilization of the precision, as well as to enable the improvement of the durability, and the like, a method for manufacturing the ultrasonic transducer, and an ultrasonic flowmeter. In order to attain this object, in accordance with the present invention, the ultrasonic transducer is so configured as to include a piezoelectric element and an acoustic matching layer, wherein the acoustic matching layer is made of a dry gel of an inorganic oxide or an organic polymer, and a solid skeletal part of the dry gel has been rendered hydrophobic. With this configuration, it is possible to obtain the ultrasonic transducer having an acoustic matching layer 3 which is very lightweight and has a small acoustic impedance due to the solid skeletal part of the dry gel which has been rendered hydrophobic. Further, it is also possible to obtain the ultrasonic transducer which shows a narrow range of characteristic variations, and is stable due to the high homogeneity of the dry gel.
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1. A method for manufacturing an ultrasonic transducer comprising an acoustic matching layer made of a dry gel of an inorganic oxide or an organic polymer, the solid skeletal part of the dry gel having been rendered hydrophobic, a gas shielding case having an inner surface and an outer surface, and a piezoelectric element mounted on said inner surface of said gas shielding case, the method comprising:
applying a gel raw material solution to the outer surface of said gas shielding case to thereby deposit said gel raw material solution on said outer surface,
said outer surface of said gas shielding case and said gel raw material solution comprising components capable of chemically bonding with one another;
solidifying the gel raw material solution to obtain a wet gel; and
removing solvent in from the wet gel to obtain a dry gel acoustic matching layer.
3. A method for manufacturing an ultrasonic transducer comprising an acoustic matching layer made of a dry gel of an inorganic oxide or an organic polymer, the solid skeletal part of the dry gel having been rendered hydrophobic, a hermetically sealed case having an inner surface and an outer surface, and a piezoelectric element mounted on said inner surface of said hermetically sealed case, the hermetically sealed case having an acoustic matching layer mounting part in concave form with a depth which is a quarter of the ultrasonic oscillation frequency at said outer surface, the method comprising:
applying a gel raw material solution to the outer surface of said hermetically sealed case to thereby deposit said gel raw material solution on said outer surface,
said outer surface of said hermetically sealed case and said gel raw material solution comprising components capable of chemically bonding with one another;
solidifying the gel raw material solution to obtain a wet gel; and
removing solvent from the wet gel to obtain a dry gel acoustic matching layer.
2. The method for manufacturing an ultrasonic transducer according to
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1. Field of the Invention
The present invention relates to an ultrasonic transducer for transmitting and receiving ultrasonic waves, and a method for manufacturing the ultrasonic transducer, and an ultrasonic flowmeter using the ultrasonic transducer.
2. Description of Prior Art
In recent years, an ultrasonic flowmeter which measures the time of flight of an ultrasonic wave across the propagation path, and determines the passing speed of a fluid, thereby to measure the flow rate has come into use for a gas meter or the like.
The operation is as follows. Upon application of an alternating voltage with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer 101 to the piezoelectric vibrator, the ultrasonic transducer 101 emits an ultrasonic wave into an external fluid along a propagation path denoted by L1 in the diagram. Then, the ultrasonic transducer 102 receives the propagated ultrasonic wave, and converts it into a voltage. Subsequently, the ultrasonic transducer 102 is used as an ultrasonic transmitter, and the ultrasonic transducer 101 is used as an ultrasonic receiver. Upon application of an alternating voltage with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer 102 to the piezoelectric vibrator, the ultrasonic transducer 102 emits an ultrasonic wave into the external fluid along a propagation path denoted by L2 in the diagram. Then, the ultrasonic transducer 101 receives the propagated ultrasonic wave, and converts it into a voltage.
Further, with such an ultrasonic transducer, if an alternating voltage is successively applied thereto, ultrasonic waves are successively emitted from the ultrasonic transducer. Accordingly, it becomes difficult to determine the time of flight. For this reason, in general, a burst voltage signal using a pulse signal as a carrier wave is used as a driving voltage. Hereinafter, the measurement principle will be described in more details. Upon application of a burst voltage signal for driving to the ultrasonic transducer 101, an ultrasonic pulse wave is emitted from the ultrasonic transducer 101. The ultrasonic pulse wave propagates through the propagation path L1 with a length L, and reaches the ultrasonic transducer 102 after (time of flight) t hours. With the ultrasonic transducer 102, the propagated ultrasonic pulse wave can be converted into an electrical pulse wave at a high S/N ratio. By using the electrical pulse wave as a trigger signal, the ultrasonic transducer 101 is driven again to emit an ultrasonic pulse wave. This device is referred to as a sing-around device. The time required for an ultrasonic pulse to be emitted from the ultrasonic transducer 101, and propagate through the propagation path to reach the ultrasonic transducer 102 is referred to as a sing-around period. The inverse thereof is referred to as a sing-around frequency.
In
f1=1/t1=(C+V cos θ)/L (1)
where t1 denotes the sing-around period which is the time for an ultrasonic pulse emitted from the ultrasonic transducer 101 to reach the ultrasonic transducer 102, and f1 denotes the sing-around frequency.
In contrast, when the ultrasonic transducer 102 is used as a transmitter, and the ultrasonic transducer 101 is used as a receiver, the following equation (2) holds:
f2=1/t2=(C−V cos θ)/L (2)
where t2 denotes the sing-around period therefor, and f2 denotes the sing-around frequency.
Accordingly, the frequency difference Δf between both the sing-around frequencies is expressed as the following equation (3), so that the flow velocity V of the fluid can be determined from the length L of the propagation path for the ultrasonic wave, and the frequency difference Δf:
Δf=f1−f2=2 V cos θ/L (3)
Namely, it is possible to determine the flow velocity V of the fluid from the length L of the propagation path for the ultrasonic wave, and the frequency difference Δf. Therefore, it is possible to determine the flow rate from the flow velocity V.
Such an ultrasonic flowmeter is required to have a high degree of precision. In order to improve the precision, the acoustic impedance of a matching layer becomes important which is formed on the transmitting and receiving surface of ultrasonic waves in the piezoelectric vibrator configuring the ultrasonic transducer for transmitting ultrasonic waves to a gas, or receiving the ultrasonic waves propagated through the gas. The acoustic impedance of the piezoelectric vibrator for generating the ultrasonic vibrations is about 30×106. The acoustic impedance of air is about 400. The ideal value of the acoustic impedance of the acoustic matching layer is about 0.11×106. Further, the acoustic impedance is defined as the following equation (4):
Acoustic impedance=(density)×(sound velocity)
Therefore, a low density material, such as a material obtained by solidifying a glass balloon or a plastic balloon with a resin material, is used for the acoustic matching layer for controlling the acoustic impedance at a low level. Alternatively, there has been adopted a method in which a hollow glass ball is thermally compressed, a molten material is foamed, or the like. The method is disclosed in Japanese Patent Publication No. 2559144, or the like.
For the acoustic matching layer used in a conventional ultrasonic transducer used for an ultrasonic flowmeter, there has been adopted a method in which a hollow glass ball is thermally compressed, a molten material is foamed, or the like, as described above. For this reason, there occur the following problems. The medium tends to be heterogeneous due to fracture of the glass ball under pressure, separation under insufficient pressure, foaming of the peeled molten material, or the like. Accordingly, variations occur in characteristics, which then generates variations in device precision. Further, there also occur the following problems. For example, since the acoustic matching layer is exposed to a gas, the surface is collapsed by the moisture, or the layer is easily deteriorated by a chemically active substance, resulting in inferior durability.
The present invention has been completed for solving such problems. It is an object of the present invention to provide a high sensitivity ultrasonic transducer, which is so configured as to reduce the variations in characteristics, thereby to enable the stabilization of the precision, as well as to enable the improvement of the durability, and the like, a method for manufacturing the ultrasonic transducer, and an ultrasonic flowmeter.
An ultrasonic transducer of the present invention is so configured as to include a piezoelectric element and an acoustic matching layer, wherein the acoustic matching layer is made of a dry gel of an inorganic oxide or an organic polymer, and the solid skeletal part of the dry gel has been rendered hydrophobic. With this configuration, it is possible to obtain the ultrasonic transducer having an acoustic matching layer which has a low acoustic impedance due to the solid skeletal part of the dry gel which has been rendered hydrophobic. Further, the ultrasonic transducer shows a narrow range of characteristic variations due to the high homogeneity of the dry gel.
Further, if the ultrasonic transducer of the present invention is embodied in the following manner, it is possible to obtain more preferred ultrasonic transducers.
First of all, the ultrasonic transducer is so configured that the piezoelectric element and the acoustic matching layer are chemically bonded with each other.
Secondly, the ultrasonic transducer is so configured that the piezoelectric element is mounted on the inner side of a hermetically sealed case, and the acoustic matching layer is mounted on the outer side of the hermetically sealed case opposed to the mounting position of the piezoelectric element.
Thirdly, the ultrasonic transducer is so configured that the hermetically sealed case has an acoustic matching layer mounting part in the form of recess with a depth which is a quarter of the ultrasonic oscillation frequency at the position of the outer side opposed to the mounting position of the piezoelectric element, and the acoustic matching layer mounting part is filled with the dry gel of an inorganic oxide or an organic polymer.
Fourthly, the ultrasonic transducer is so configured that the hermetically sealed case and the acoustic matching layer are chemically bonded with each other.
Fifthly, the ultrasonic transducer is so configured that the hermetically sealed case is made of a conductive material.
Sixthly, the ultrasonic transducer is so configured that the conductive material is a metal material.
Seventhly, the ultrasonic transducer is so configured that the dry gel constituting the acoustic matching layer has a density of 500 kg/m3 or less, and a mean pore diameter of 100 nm or less.
Eighthly, the ultrasonic transducer is so configured that the solid skeletal part of the dry gel contains at least silicon oxide or aluminium oxide as a component.
Ninthly, the ultrasonic transducer is so configured that a protective layer with a density of 800 kg/m3 or more, and a thickness of 10 μm or less is formed on the surface of the acoustic matching layer.
Tenthly, the ultrasonic transducer is so configured that the protective layer is made of any of a metal material, an inorganic material, and a polymer material.
Eleventhly, the ultrasonic transducer is so configured that the protective layer is made of any of aluminium, silicon oxide, aluminium oxide, amorphous carbon, and polyparaxylene.
In a method for manufacturing an ultrasonic transducer of the present invention, the ultrasonic transducer includes an acoustic matching layer made of a dry gel of an inorganic oxide or an organic polymer, the solid skeletal part of the dry gel having been rendered hydrophobic, and a piezoelectric element. The method includes a step of brazing (or soldering) the dry gel to the piezoelectric element or a gas shielding case on the inner side of which the piezoelectric element is mounted. With the ultrasonic transducer obtained by using this manufacturing method, it is possible to achieve higher sensitivity and stabilization of the characteristics due to the acoustic matching layer with a low acoustic impedance.
In a method for manufacturing an ultrasonic transducer of the present invention, the ultrasonic transducer includes an acoustic matching layer made of a dry gel of an inorganic oxide or an organic polymer, the solid skeletal part of the dry gel having been rendered hydrophobic, and a piezoelectric element. The method includes a step of forming the acoustic matching layer. The acoustic matching layer formation process includes: a deposition step of applying a gel raw material solution to the piezoelectric element or a gas shielding case on the inner side of which the piezoelectric element is mounted; a solidification step of obtaining a wet gel from the gel raw material solution; and a drying step of removing a solvent in the wet gel to obtain a dry gel. With the ultrasonic transducer obtained by using this manufacturing method, it is possible to achieve higher sensitivity and stabilization of the characteristics due to the acoustic matching layer with a low acoustic impedance.
Further, if the method for manufacturing an ultrasonic transducer of the present invention is embodied in the following manner, it is possible to obtain more preferred ultrasonic transducers.
First of all, in the ultrasonic transducer including the piezoelectric element mounted on the inner side of a hermetically sealed case, the hermetically sealed case has an acoustic matching layer mounting part in the form of recess with a depth which is a quarter of the ultrasonic wave length at the position of the outer side opposed to the mounting position of the piezoelectric element of the hermetically sealed case. In the method for manufacturing the ultrasonic transducer, the gel raw material solution is applied to the acoustic matching layer mounting part.
Secondly, in the method for manufacturing the ultrasonic transducer, a protective layer is formed on the surface of the acoustic matching layer by a dry deposition method.
Further, an ultrasonic flowmeter of the present invention includes: a flow rate measuring part through which a fluid to be measured flows; a pair of ultrasonic transducers for transmitting and receiving an ultrasonic wave mounted at the flow rate measuring part; a measuring circuit for measuring the time of flight of an ultrasonic wave between the ultrasonic transducers; and a flow rate operation means for calculating the flow rate based on the signal from the measuring circuit, each of the ultrasonic transducers being made up of a hermetically sealed case by which the fluid to be measured and the piezoelectric element are shielded from each other. With this ultrasonic flowmeter, it is possible to achieve the improvement of the stability of the flow rate measurement due to the high sensitivity and the narrow range of variations in characteristics of the ultrasonic transducers.
As described above, the present invention provides such a configuration that the acoustic matching layer is made of a dry gel of an inorganic oxide or an organic polymer, and the solid skeletal part of the dry gel has been rendered hydrophobic. Accordingly, it is possible to obtain an ultrasonic transducer having an acoustic matching layer which is very lightweight and has a small acoustic impedance due to the solid skeletal part of the dry gel which has been rendered hydrophobic. Further, it is also possible to obtain the ultrasonic transducer which shows a narrow range of characteristic variations, and is stable due to the high homogeneity of the dry gel. Still further, upon formation of the dry gel of an inorganic oxide or an organic polymer, the OH group on the piezoelectric element surface or the case surface and the component of the raw material react and chemically bonded with each other to ensure the bond therebetween. Therefore, such an excellent effect can also be expected that an adhesion layer-free, or a so-called adhesion layer-less ultrasonic transducer is obtainable.
Such objects and advantages of the present invention will become more apparent from the following description of embodiments given by reference to the accompanying drawings.
The protective layer 5 is a layer with a density of 800 kg/m3 or more, and a thickness of 10 μm or less, and made of a metal material, an inorganic material, a polymer material, or the like. Specifically, other than aluminium, silicon oxide, aluminium oxide, low melting glass, amorphous carbon, polymers (polyparaxylene and polyimide), and the like, coating resins, UV (ultraviolet) curable resins, thermosetting resins, and the like are used. Further, when aluminium is used, it is provided through a vapor deposition or sputtering process. When silicon oxide or aluminium oxide is used, it is provided through a vapor deposition process, a sputtering process, a CVD (Chemical Vapor Deposition) process, or the like. When low melting glass is used, it is provided through a coating process. When an amorphous carbon is used, it is provided through a plasma CVD process. When a polymer (polyparaxylene or polyimide) is used, it is provided through a vapor deposition polymerization process, or the like.
With the ultrasonic transducer thus configured, when a pulse wave voltage having an AC signal component with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer is applied to the driving terminals 6a and 6b, the piezoelectric vibrator 1 vibrates in thickness vibration mode. Accordingly, it emits burst-like ultrasonic waves into a fluid such as a gas or a liquid.
Then, a description will be given to examples of a method for manufacturing each of the acoustic matching layers 3 shown in
Step 1: electrodialysis of sodium silicate is performed to form a silicic acid solution with a pH of 9 to 10;
Step 2: the silicic acid solution is adjusted to a pH of 5.5, and the resulting solution is poured into a mold. Accordingly, the silicic acid solution gels with time to obtain a water base wet gel block;
Step 3; the gelled silicic acid solution is rendered hydrophobic by an acetone solution of trimethylchlorosilane (TMCS), followed by a dehydration treatment, to form a wet gel block;
Step 4: the wet gel block is subjected to supercritical. drying by using carbon dioxide to form a silica dry gel block;
Step 5: the silica dry gel block is cut to a thickness of λ/4 to form a prescribed acoustic matching layer 3; and
Step 6: the acoustic matching layer 3 cut in the step 5 is bonded to one side of the piezoelectric element 2 or the top 14a of the case 14 through silver soldering or an adhesive, resulting in the piezoelectric vibrator 1.
Step 1: a sol solution is formed from tetraethoxysilane, aluminium-sec-buthoxide, and ethanol;
Step 2: to the resulting sol solution, a gelling catalyst, or an acid or a base is added, to form a coating raw material solution before start of gelation, to be coated in its thickened state to the case 14;
Step 3: the coating raw material solution is coated to the coating surface of the case 14, and merged with the case (hermetically sealed case) 14;
Step 4: when the coating raw material solution is allowed to gel after coating, the OH group on the surface of the case 14 and the alkoxy group of the raw material react and are chemically bonded with each other to form a wet gel film on the surface of the case 14;
Step 5: the film is washed with ethanol. After washing, the film is supercritically dried. Subsequently, it is subjected to a hydrophobization treatment with the vapor of trimethylchlorosilane to form an aluminosilica dry gel film on the surface of the case 14;
Step 6: On the aluminosilica dry gel film formed on the surface of the case 14, a dry protective film (protective layer 5) is formed in vacuum. The protective film 5 is formed by vapor deposition, sputtering, CVD, or the like of silicon dioxide; and
Step 7: thereafter, to the case 14, the piezoelectric element 2, the cover plate 7, the driving terminals 6a and 6b, and the like are mounted, resulting in the piezoelectric vibrator 1.
Step 1: electrodialysis of sodium silicate is performed to form a silicic acid solution with a pH of 9 to 10;
Step 2: the silicic acid solution is adjusted to a pH of 5.5, and the resulting solution is added dropwise onto the case 14 for merging between the silicic acid solution and the case 14;
Step 3: when the coating raw material solution is allowed to gel after coating of the coating raw material solution, the OH group on the surface of the case 14 and the alkoxy group of the raw material react and are chemically bonded with each other to form a wet gel film on the surface of the case 14;
Step 4: the gelled silicic acid solution is rendered hydrophobic by an acetone solution of trimethylchlorosilane (TMCS), followed by a dehydration treatment. Further, after solvent exchange to hexane, the solution is dried in a case held at 100° C. to form a silica dry gel film of silicon dioxide;
Step 5: on the silica dry gel film formed on the surface of the case 14, a protective film (protective layer 5) of amorphous carbon (diamond-like carbon) film is formed by high frequency plasma CVD. The protective film 5 is hard, and resistant to scratch, and has a high chemical resistance. Further, it is excellent in gas barrier property, and it has a low sorption ability, and is less likely to sorb chemical substances; and
Step 6: thereafter, to the case 14, the piezoelectric element 2, the cover plate 7, the driving terminals 6a and 6b, and the like are mounted, resulting in the piezoelectric vibrator 1.
Step 1: tolylene diisocyanate (TDI) and toluene diamine (TDA) are mixed in an aprotic solvent such as acetone or tetrahydrofuran to form an aqueous solution having a polyurea structure. It is important that a solvent which will not react with the raw material is prepared as the solvent in this step;
Step 2: the resulting aqueous solution is added dropwise onto the case 14 to be merged with the case 14;
Step 3: when the coating raw material solution is allowed to gel after coating of the coating raw material solution, the OH group on the surface of the case 14 and the alkoxy group of the raw material react and are chemically bonded with each other to form a wet gel film on the surface of the case 14;
Step 4: the gelled coating raw material solution is subjected to solvent exchange to tertiary butyl alcohol, and then frozen at 25° C. or less. Subsequently, the frozen solution is heated and dried under reduced pressure of 41 Torr or less to form an organic dry gel film. The resulting organic dry gel film has water repellency;
Step 5: on the organic dry gel film formed on the surface of the case 14, an urethane-based ultraviolet (UV) curable resin is coated, and cured to form a protective film (protective layer 5); and
Step 6: thereafter, to the case 14, the piezoelectric element 2, the cover plate 7, the driving terminals 6a and 6b, and the like are mounted, resulting in the piezoelectric vibrator 1.
The operation of the ultrasonic flowmeter thus configured will now be described below. The fluid to be measured is set to be, for example, a LP gas, and the driving frequency of the ultrasonic transducers 1a and 1b is set to be about 500 kHz. The control unit 59 outputs a transmission start signal to the driving circuit 54, and at the same time, starts the time measurement of the timer 57. The driving circuit 54 receives the transmission start signal, and drives the ultrasonic transducer 1a to transmit an ultrasonic pulse. The transmitted ultrasonic pulse propagates in the flow rate measuring part, and is received at the ultrasonic transducer 1b. The received ultrasonic pulse is converted into an electric signal at the ultrasonic transducer 1b, and outputted to the reception and detecting circuit 56. The reception and detecting circuit 56 determines the reception timing of the reception signal, and stops the timer 57, and the operation unit 58 performs operation on the time of flight t1.
Subsequently, the switching circuit 55 switches between the ultrasonic transducers 1a and 1b connected therethrough to the driving unit 54 and the reception and detecting circuit 56. Then, the control unit 59 again outputs a transmission start signal to the driving circuit 54, and at the same time, starts the time measurement of the timer 57. Contrary to the case of measurement of the time of flight t1, the ultrasonic transducer 1b transmits an ultrasonic pulse, and the ultrasonic transducer 1a receives it, and the operation unit 58 performs operation on the time of flight t2.
Herein, assuming that the distance connecting between the centers of the ultrasonic transducers 1a and 1b is L, the sound velocity of the LP gas in airless conditions is C, the flow velocity in the flow rate measuring part 51 is V, and the angle between the direction of flow of the fluid to be measured and the line connecting between the centers of the ultrasonic transducers 1a and 1b is θ, it is possible to determine the times of flight t1 and t2, respectively. Further, since the distance L is known, it is possible to determine the flow velocity V if the times t1 and t2 are determined. Accordingly, it becomes possible to check the flow rate from the flow velocity V.
Having described the present invention as related to the preferred embodiments shown in the accompanying drawings, it will be obvious to those skilled in the art that changes and modifications may be made with ease to the invention. Such variations are intended to, be within the scope of the present invention.
Suzuki, Masaaki, Hashimoto, Masahiko, Hashida, Takashi
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